thanks for your reply.
since posting my question i have individualy weighed a batch of these remmington subsonics with lee reloading scales and found that there is in most cases a 30% deviation in the amount of power in the shells with the conclusion that the light loads werent igniteing.
i had changed the firing pin as i said, possibly you could be on to somthing with the springs as they havnt been replaced to my knowledge.
The rifle was made in 1948 so i'd say it has seen a few shoots
il try and source a new spring or entire bolt and see how i go.